The term "polyphenylene ether" resin is descriptive of a well-known group of polymers that may be made by a variety of catalytic and non-catalytic processes.
The polyphenylene ethers are known and described in numerous publications, including U.S. Pat. Nos. 3,306,874 and 3,306,875 (Hay); and U.S. Pat. Nos. 3,257,357 and 3,257,358 (Stamatoff). They are useful for many commercial applications requiring high temperature resistance and, because they are thermoplastic, they can be formed into films, fibers and molded articles. In spite of these desirable properties, parts molded from polyphenylene ethers are somewhat brittle and exhibit poor impact strength. In addition, the relatively high melt viscosities and softening points are considered disadvantages in many uses. Films and fibers can be formed from polyphenylene ethers on a commercial scale using solution techniques, but melt processing is commercially unattractive because of the high temperatures required to soften the polymer and the problems associated therewith, such as instability and discoloration. Such techniques also require specially designed process equipment to operate at elevated temperatures. Molded articles can be formed by melt processing techniques, but again, the high temperatures required are undesirable.
In addition, although the polyphenylene ether resins have outstanding hydrolytic stability, making them very useful in contact with aqueous media, e.g. in dishwasher and laundry equipment, they will soften or dissolve when placed in contact with many other, more aggressive, solvents, e.g., halogenated or aromatic hydrocarbons and gasoline, which limits their use in applications where exposure to such solvents does or may occur.
It is known in the art that the properties of the polyphenylene ethers can be materially altered by forming compositions with other polymers and many such polyphenylene ether molding compositions have been disclosed in the prior art.
Blends of polyphenylene ethers and polyolefins are of great interest because they can bring some of the chemical resistance of the polyolefins to the polyphenylene ethers. However, such blends very often suffer from limitations of their own, apart from the limitations of the individual resins, due to the relative incompatibility of the polyphenylene ether resin and the polyolefin resin, especially in those blends where one of the resins is present in an amount of more than about 10% by weight, based on 100% by weight of the two combined. In such instances, the beneficial property aspects which one resin could possibly confer on the other may not be fully realized.
For instance, in a commonly-assigned patent, U.S. Pat. No. 3,361,851 (Gowan), polyphenylene ethers are formed into compositions with polyolefins to improve impact strength and resistance to aggressive solvents. Gowan discloses that the polyolefin may comprise from 1-10% by weight of the total composition.
One serious limitation of such systems is a severe tendency to undergo delamination, due to the agglomeration of dispersed domains into larger particles, thereby causing deterioration of physical properties. Another limitation is brittleness of the composition. The prior art has sought to improve the properties of these blends. S-EB-S rubbers are commonly used to improve the compatibility of the polyphenylene ether and polyolefin resins and to increase morphological stability in the melt. These systems are relatively inefficient because they require high levels of rubber to give a substantial effect.
For instance, in U.S. Pat. No. 4,116,055 (Lee, Jr.), compositions comprising a polyphenylene ether, an elastomeric block copolymer of a vinyl aromatic compound and a conjugated diene, and a polyolefin, optionally containing a styrene homopolymer or random copolymer resin, having improved toughness over compositions containing either the block copolymer or the polyolefin resin alone are disclosed. The polyolefin resin plus block copolymer of the A-B-A type is present at about 5-30% by weight of the total composition. The center block B is a conjugated diene of higher molecular weight than the combined weight of terminal blocks A and A'.
In U.S. Pat. No. 4,383,082 (Lee, Jr.), it is disclosed that larger amounts of polyolefin resin can be successfully incorporated in a polyphenylene ether resin when certain elastomeric diblock copolymers of the A-B type comprising an alkenyl aromatic compound A and a conjugated diene B, or a radial teleblock copolymer of an alkenyl aromatic compound and a conjugated diene, are used.
It is therefore an object of the present invention to provide polyphenylene ether resin/polypropylene resin compositions which have improved physical property profiles over prior art compositions.
It has now been surprisingly discovered that if a polyphenylene ether resin having a very low intrinsic viscosity, as that term is described hereinafter, is utilized in polymeric resin compositions comprised of polyphenylene ether resin and polypropylene resin, the heat deflection temperature (HDT) and stiffness are improved significantly over similar compositions which utilize "conventional" higher intrinsic viscosity polyphenylene ether resins. In addition, minor amounts, i.e. less than about (20%) of an impact modifier such as a diblock or triblock copolymer comprised of an alkenyl aromatic resin and a conjugated diene, or such as a rubber-modified alkenyl aromatic resin, can be employed in the present compositions. Significantly, the improvements in heat deflection temperature and stiffness, relative to comparison compositions, are maintained in the impact modified compositions and, in some instances, are even more improved.